1. Field of the Invention
The present invention relates to a write-once type optical disk recording/reproducing apparatus, and more particularly to a laser driver circuit for supplying current to a laser diode of an optical pickup.
2. Description of the Related Art
There is known a write-once (WO) type optical disk recording/reproducing apparatus which can write data only once in an optical disk using a laser beam.
With such an optical disk recording/reproducing apparatus, a laser beam is modulated by an EFM signal and applied to an optical disk to write a train of bits. In reproducing data, a laser beam having a constant low power is applied to a recorded bit train, and the reflected laser beam from the bit train is converted into current to reproduce the recorded data.
The current/output characteristic of a laser diode used for such an optical disk recording/reproducing apparatus changes with an ambient temperature or its own temperature under a long period of operation. A desired laser beam output cannot be obtained therefore by simply controlling a current to be supplied to a laser diode.
In view of this, the following approach has been used heretofore. Namely, a light receptor such as a photodiode is disposed near a laser diode. A current to be supplied to the laser diode is controlled such that a current of the light receptor generated upon reception of a laser beam becomes constant.
An example of a conventional laser driver circuit using such an approach is shown in FIG. 6.
In FIG. 6, reference numeral 1 represents a microcomputer which generates control signals such as LDC and REC signals for the control of a laser diode (LD).
Reference numeral 2 represents a transistor which supplies current to the laser diode LD. The collector of the transistor 2 is connected to a positive voltage source, and the emitter thereof is connected via a resistor R1 to the Anode of the laser diode LD.
Reference numeral 7 represents a constant current source for supplying a base current to the transistor 2. A current from the constant current source 7 flows distributively into the base of the transistor 2, a resistor R3, and the collector of a transistor 8.
The emitter of the transistor 8 is grounded, and the base thereof is grounded via a resistor and connected via another resistor to an LDC output terminal of the microcomputer 1.
The resistor R3 is connected to the collector of a transistor 6. The emitter of the transistor 6 is connected to a negative voltage source, and the base thereof is connected via a resistor to an output terminal of an operational amplifier 4.
A laser beam from the laser diode is received by a photodiode (PD) whose anode is grounded and whose cathode is connected to the negative voltage source via a parallel circuit of variable resistors VR3 and VR4. Between the variable resistor VR4 and the anode of the photodiode PD, there is connected an analog switch 3 which is opened when the REC signal outputted from the microcomputer 1 takes a low level (L), and closed when it takes a high level (H).
The anode of the photodiode PD is connected via a serial circuit of a resistor R2 and a capacitor C1 to the negative voltage source. The interconnection between the resistor R2 and the capacitor C1 is connected to the non-inverting input terminal of the operational amplifier 4. Connected between the inverting input terminal of the operational amplifier 4 and the negative voltage source is a constant voltage source 5.
The interconnection between the resistor R1 and the laser diode LD is connected to the collector of a transistor 11. The emitter of the transistor 11 is grounded via a resistor, and the base thereof is connected via a resistor to the movable contact of a variable resistor VR2.
The variable resistor VR2 is connected between the ground and the emitter of a transistor 10.
The collector of the transistor 10 is connected to the positive voltage source, and the base thereof is connected via a resistor to the output terminal of a NOR gate 9.
Inputted to the NOR gate 9 are REC and LDC signals from the microcomputer 1 and a recording date (EFM signal).
During a reproducing mode of the above-described laser driver circuit, the REC signal of the microcomputer 1 takes "H" level, and the LDC signal takes "L" level. The output of the NOR gate 9 becomes "L" accordingly, and the transistors 10 and 11 turn off. The emitter current of the transistor 2 will not distributively flow to the transistor 11, but only to the laser diode LD. In this case, the analog switch 3 is maintained open, and the transistor 8 is maintained turned off.
The laser diode LD radiates a laser beam whose light amount corresponds to the current flowing therethrough. The photodiode PD receiving a laser beam causes a current to flow to the variable resistor VR3, the current corresponding to the light amount of the laser beam.
A voltage generated across the variable resistor VR3 by the photodiode current is applied to the non-inverting input terminal of the operational amplifier 4. A voltage at the output terminal of the operational amplifier 4 is regulated such that the voltage applied to the non-inverting input terminal becomes equal to that of the constant voltage source 5.
The voltage at the output terminal of the operational amplifier 4 controls the collector current of the transistor 6, and hence the current distributively flowing to the resistor R3.
In the above manner, during the reproducing mode, a current to be supplied to the laser diode LD is controlled such that the light amount outputted from the laser diode LD becomes constant as defined by the variable resistor VR3.
During the recording mode, the REC and LDC signals from the microcomputer 1 takes and "L" level.
As a result, the analog switch 3 is maintained closed, and the transistor 8 is maintained turned off.
An output of the NOR gate 9 takes an "L" and "H" level as the recording data (EFM signal) takes an "H" and "L" level, respectively. The current flowing through the resistor R1 distributively flows to the transistors 10 and 11 when the output of the NOR gate 9 takes an "H" level, so that the current flowing through the laser diode LD is modulated by the EFM signal at a duty ratio 0.5.
The bottom current of the laser diode LD can be adjusted by the variable resistor VR2.
The laser diode LD radiates a laser beam whose light amount corresponds to the current flowing therethrough. The photodiode PD receiving a laser beam causes a current to flow to the variable resistors VR3 and VR4, the current magnitude corresponding to the light amount of the laser beam.
A voltage generated by the current across the variable resistors VR3 and VR4 is applied to the non-inverting input terminal of the operational amplifier 4 after its d.c. components are removed by a smoothing circuit of the resistor R2 and the capacitor C1.
A voltage at the output terminal of the operational amplifier 4 is regulated such that the voltage applied to the non-inverting input terminal becomes equal to that of the constant voltage source 5.
The voltage at the output terminal of the operational amplifier 4 controls the collector current of the transistor 6, and hence the current distributively flowing to the resistor R3.
In the above manner, during the recording mode, a current to be supplied to the laser diode LD is controlled such that the average light amount outputted from the laser diode LD becomes constant as defined by the variable resistors VR3 and VR4.
Neither in the recording mode nor in the reproducing mode, the LDC signal takes an "H" level so that the transistor 8 causes to cut the base current to the transistor 2 and no current will be supplied to the laser diode LD.
With the conventional laser driver circuit described above, it is possible to control the laser output light amount during a steady state in the recording or reproducing mode. However, there is a delay in detecting a laser beam output at the laser output control loop. There arise therefore some problem that the waveform of a laser output has a delayed leading edge or an overshoot when the operation changes from the reproducing mode to the recording mode.